In CVT, for example, a pair of pulleys are attached to a driving rotation shaft and a driven rotation shaft, and an endless belt engages both pulleys to connect both rotation shafts. The driving pulley has a variable sheave and a fixed sheave, and the driven pulley also has a variable sheave and a fixed sheave. An oil chambers is mounted to the variable sheave of the pulley attached to each of the driving and driven shafts. By operation pressure applied to each oil chamber, the variable sheaves of both pulleys move axially, varying the groove widths of both pulleys for controlled speed change. A seal ring is disposed in a seal ring groove of an outer peripheral surface of an annular inner member (for example, a partition member constituting the oil chamber on the rear surface side of the variable sheave). The seal ring has a side surface and an inner peripheral surface for receiving the pressure of an oil supplied from an oil pump, and an opposite side surface and an outer peripheral surface for securing sealing between the side surface of the seal ring groove and an inner peripheral surface of an outer member (for example, a cylindrical portion of the variable sheave). The inner member and the outer member are slidable via the seal ring to change the groove width of the pulley. Namely, the outer peripheral surface of the seal ring is axially slidable to the inner peripheral surface of the outer member.
Required to provide the seal ring with excellent sealability are strict specifications, such as improved durability and reliability for higher performance of engines, the size and weight reduction of oil pumps for energy saving, and stabilized oil leakage for more accurate electronic control. An ultimate target is to avoid oil leakage even in a state where an oil pump is not operated by the stop of an engine (oil pressure is not applied to the seal ring).
For example, as shown in FIG. 8, a combination of a gap-free endless seal ring 101 of polytetrafluoroethylene (PTFE) and an O-ring 102 of a synthetic rubber can make oil leakage zero, in a state where oil pressure is not applied. However, the above seal ring suffers the following disadvantages in the assembling efficiency of CVT. Two operations should be carried out to expand both O-ring and PTFE seal ring separately to assemble them in a groove from an end of the inner member. Also, the diameter reduction of the PTFE seal ring to be inserted into the outer member is not easy because of the O-ring disposed inside, resulting in an extremely inefficient assembling step.
Attempts to improve the assemblability of a gap-free seal ring in a groove of an inner member are proposed by JP 2008-190643 A and JP 2008-190650 A. JP 2008-190643 A discloses an endless seal ring having pluralities of axially slanting ridges extending on its inner peripheral surface, tip ends of these ridges being in contact with a sealing groove bottom, and each ridge being circumferentially deformed when passing a sealing groove edge, thereby making it easy to assemble the endless seal ring to the sealing groove. JP 2008-190650 A discloses a seal ring having an outer portion made of a resin and an inner portion made of an elastic material, the inner elastic material portion intruding into the outer resin portion, such that the outer resin portion having large deformation resistance has a small cross section area, while the inner portion of an easily deformable elastic material has a large cross section area, for improved assemblability.
However, because each seal ring of JP 2008-190643 A and JP 2008-190650 A is designed to have an inner peripheral surface in contact with a sealing groove bottom, there is large assembling resistance when the seal ring is inserted into the outer member, though the seal ring can be easily assembled in the inner member groove. Thus, a special jig should be devised, and there actually remain a lot of problems to be solved.